The prior art of the present invention will be described below by using the drawings. FIG. 8 is a view showing the configuration of a conventional ultrasonic probe (for example, see the following patent document 1). In the explanation of this drawing, [UP] and [DOWN] are assumed to indicate the upward direction and the downward direction in the plane of the drawing paper, respectively. In FIG. 8, a piezoelectric element 81 having electrodes on both surfaces is an element for transmitting and receiving ultrasonic waves. On the upper surface of the piezoelectric element 81, a ground electrode layer 82 is formed as a first electrode layer, and on the lower surface, a positive electrode layer 83 is formed as a second electrode layer, respectively, in advance.
A conductive acoustic matching layer 85 is intended to efficiently transmit and receive the ultrasonic waves to a test subject (living body) and layered on the upper surface (the side of the ground electrode layer 82) of the piezoelectric element 81, via a compressively-cured insulating adhesive layer 84. On the acoustic matching layer 85, a polymer film 87 made of polyimide and the like is layered via the compressively-cured insulating adhesive layer 84. The polymer film 87 is composed of the two layers of a film body 88 as an acoustic matching layer and a conductor layer (copper layer) 89 formed on the side of an adhesive layer 86 of this film body 88. Although not shown in FIG. 8, in order to further attain the acoustic matching with the living body, there is a case that a second acoustic matching layer made of a polymer material and the like is layered via an adhesive layer, on the surface of the film body 88. Moreover, an acoustic lens 90 is placed on the surface of the second acoustic matching layer.
By the way, the plurality of acoustic matching layers, which are layered on the piezoelectric element 81 and the piezoelectric element 81, are divided into a plurality of arrays, which are electrically independent of each other, by dicing. On the lower surface (the side of the positive electrode layer 83) of the piezoelectric element 81, an FPC 811 is layered via a compressively-cured insulating adhesive layer 810. In this FPC 811, a base unit 812 made of polyimide is formed, and a conductive pattern 813 corresponding to the piezoelectric element 81 is formed on the side of the piezoelectric element 81 on this base unit 812. Also, both sides of the FPC 811 protrude from the layered portion of the piezoelectric element 81, and electrode extracting units 811a are formed on both ends of the protrusions. A backing material 814 is attached via an adhesive layer 815 to the side of the base unit 812 of the FPC 811 and mechanically supports the piezoelectric element 81 and acoustically brakes the piezoelectric element 81 and consequently makes an ultrasonic pulse waveform short.
In the conventional ultrasonic probe, the ground electrode layer 82 in the piezoelectric element 81 and the conductive acoustic matching layer 85, and the acoustic matching layer 85 and the polymer film 87 on which the conductive pattern is formed, and the positive electrode layer 83 in the piezoelectric element 81 and the conductive pattern 813 in the FPC 811 are electrically connected, respectively, because insulating epoxy resins are pressed, heated and cured and consequently adhered in very thin manners. Irrespectively of the existence of the thin insulating layer, a tunnel effect, a Schottky conduction, an impurity conduction, or the like causes the charges from being shifted. When the insulating layer is uniformly thin, the conduction resistance is constant. However, the thicknesses of the insulating adhesive layers 84, 86 and 810 are not constant. Thus, with the surface roughness of the opposite conductor member, a conduction resistance exists under a certain probability.
Depending on a place, the thickness is very close to 0. In short, there are the portions where the ground electrode layer 82 in the piezoelectric element 81 and the acoustic matching layer 85, and the acoustic matching layer 85 and the conductive layer 89 in the polymer film 87, and further the positive electrode layer 83 in the piezoelectric element 81 and the conductive pattern 813 in the FPC 811 are in contact, respectively. Thus, the conduction resistances of those contact portions are small. Hence, depending on the adhesion state between the materials, namely, the total area of the contact portions between the mutual materials, the conduction resistance is changed.
The piezoelectric element 81, the conductive acoustic matching layer 85, the polymer film 87 where the conductive pattern is formed, and the conductive pattern of the FPC 811, which are adhesively layered via the insulating epoxy resins, are divided into the plurality of arrays, which are electrically independent of each other, by dicing. However, the conduction resistances of the electrode connectors are irregular in the respective arrays. As a result, there was a problem that the sensitivities were irregular between the arrays. A higher frequency orientation makes its problem severer, in recent years in which the width of the division electrode is narrower.
Also, as a method of solving the foregoing problems, there is a method of using a conductive paste, or an anisotropic conductive adhesive that has a conductivity only in a thickness direction, instead of the adhesive of the insulating epoxy resin.    Patent Document 1: Japanese Patent No. 3423788 (Pages 1 and 2, FIG. 2)
However, in the conductive paste that exhibits the conductivity because a conductive path is generated by the volume contraction caused by the thermal curing or by the low temperature sintering between metal powder, or in the anisotropic conductive adhesive in which micro conductive particles are uniformly distributed in a one-component type adhesive that is high in insulation, its curing temperature is high such as 100 deg C. or more. When the polarization-treated piezoelectric element is placed under the high temperature environment such as 100 deg C. or more, the piezoelectric property is lost, which causes the practical use of the conductive paste or anisotropic conductive adhesive from being prevented. Also, the conductive paste and the anisotropic conductive adhesive have the problem that they are expensive.